Heretofore, a fuel cell system including a fuel cell for receiving a supplied reactant gas (a fuel gas and an oxidizing gas) to generate a power has been put to practical use. When the fuel cell system generates the power, water is formed in the fuel cell by an electrochemical reaction. In an environment at a low temperature below the freezing point or the like, in a case where the fuel cell is stopped and left to stand while the water remains in the fuel cell, a start performance at the next start deteriorates owing to the freezing of the water on occasion. Therefore, various technologies for decreasing a water content in the fuel cell have presently been suggested.
On the other hand, to efficiently generate the power in the fuel cell, the wet state of an electrolytic film constituting the fuel cell needs to be kept. Consequently, in recent years, a technology has been suggested in which the power generation of the fuel cell is forcibly continued while controlling the water content of the fuel cell, to keep the wet state of the fuel cell (e.g., see Japanese Patent Application Laid-Open No. 2005-26054).